How do I make two objects pull toward each other in a zero-gravity scene?
To make the your rigid bodies attract in your scene (assuming you already added your rigid bodies), you need to do this for each object:

1- Add a Force Field: In the 3d view, press ShiftA > Force Field > Force

2- Make your object the parent of the Force Field: Shift + Right click on the Force Field, followed by your object (The order is important) > Ctrl+P > Parent

3- Make sure your origin is on the center of mass of your object:
Select your object, then in the tools panel, select Set origin > Center of mass

4- Align the Force Field to your object: Click on your object > Shift+S > cursor to selected, Click on the Force Field > Shift+S > Selected to Cursor

5- Modify the settings of the Force Field: In the properties panel physics tab, you can find the settings for the force field. By deafult, the strength is set to a positive value, which will push objects apart. To make them attract, input a negative value. Note that massive/distant objects will require a lot more strength to be effective.

Repeat this process for every object in your scene, and you will have your magnetic attraction the next time you click play.